Synchrotron radiation is needed to collect higher resolution data from the LT-IIb holotoxin, both native crystals and soaked or co-crystallized with receptor analogues. If the MR structure solution has not succeeded by then we will use bromine-substituted sugars as receptor analogues, which should yield five Br sites for SIR/SAS phasing. We have constructed several active site mutants of LT, in addition to those previously crystallized. Experience to date is that these mutants produce small, radiation-sensitive, crystals which will require synchrotron radiation to produce adequate data quality.